QEMU commit 3984890 introduced the "pci-hole64-size" property,
to i440FX-pcihost and q35-pcihost with a default setting of 2 GB.

Translate <pcihole64>x<pcihole64/> to:
-global q35-pcihost.pci-hole64-size=x for q35 machines and
-global i440FX-pcihost.pci-hole64-size=x for i440FX-based machines.

Error out on other machine types or if the size was specified
but the pcihost device lacks 'pci-hole64-size' property.

https://bugzilla.redhat.com/show_bug.cgi?id=990418